The present invention relates to an automatic gain control method and apparatus in an optical disk system and, more particularly, to automatic gain control method and apparatus which can discriminate whether a data has been recorded in a certain area on an optical disk to record user's data or not.
Generally, in the optical disk system, a number of tracks are formed on an optical disk, each track is divided into a plurality of sectors, and information is recorded or reproduced on a sector unit basis. Each sector has a preformat portion in which control information such as identification (ID) and the like has previously been recorded and an information recording portion to record user's data.
Hitherto, as an optical disk apparatus having a function to discriminate whether a data has correctly been recorded in the information recording portion or not, for instance, there has been known a technique disclosed in JP-A-60-115073. According to such a technique, an envelope of a reproduction signal of the optical disk is detected, the level of the envelope signal is compared with a reference potential, a time duration of a period when the envelope signal level is deviated from the reference potential is counted, and when the time duration is shorter than the reference time duration it is determined that the data has correctly been recorded.
According to the above conventional technique, no consideration is made with respect to the signal-to-noise (S/N) ratio of the information signal and there is a problem such that even if a good S/N ratio is obtained, the signal in which the absolute level of the amplitude is small cannot be recognized as data.
That is, in the above conventional technique, since it is considered that the fluctuation in amplitude itself indicates the data quality, there can occur a case where in spite of the fact that the S/N ratio is enough high, when the absolute amplitude of the signal S is merely small, it is determined that the data quality is bad, that is, the data is not normal. As such a case, for instance, there can be mentioned a case where in spite of the fact that the recording state of the user's data for the magnetooptic disk is good, although the S/N ratio is high, the signal level S decreases due to a deterioration in sensitivity of the disk by an ambient temperature, a reflectance of the disk, a deterioration in light source upon reproduction, and a variation in efficiency of a photodetector, or the like. In such a case, in the conventional technique, the data guality which is higher than it is needed is guaranteed.
On the other hand, generally, in the optical disk recording and reproducing apparatus, an automatic gain control method whereby an amplitude fluctuation of the reproduction signal is compensated to make the amplitude constant has conventionally been well known. However, in this case, as the reproduction signal amplitude (level) decreases, a gain of an amplifier of the gain controller is increased. Thus, there is a problem such that the noise whose amplitude (level) is small and the remaining signal component which was not erased when erasing the recorded data are also enhanced to predetermined levels and they are erroneously recognized as signals.